Various types of heart valve prostheses have been developed which operate hemodynamically as a result of the pumping action of the heart. Such heart valves include valves having single occluders which pivot along an eccentric axis (or both pivot and translate), to open and close the blood flow passageway, such as those described in U.S. Pat. Nos. 3,546,711, 4,011,601, 4,425,670 and 4,725,275, and also include bi-leaflet heart valves, such as those described in U.S. Pat. Nos. 4,078,268, 4,159,543, 4,254,508, 4,276,658, 4,328,592, and 4,535,484. The above-mentioned patents illustrate a variety of different arrangements for pivotally interconnecting valve members (i.e. occluders) and valve bodies.
In recent years, hi-leaflet heart valves have generally become the mechanical valve of choice. A bi-leaflet valve typically includes a valve body or orifice ring in which occluder means is in the form of a pair of leaflets which are pivotally mounted so as to alternately open to permit blood to flow in the downstream direction and close in response to backflow in the upstream direction. The outer surface of the valve body often has an annular channel which receives a metal stiffener ring which can be used for interconnecting with and supporting a sewing ring.
Commonly used bileaflet heart valves generally include two leaflets pivotally mounted by integral, laterally extending ears within recesses formed in a valve body which provides a conduit or central passageway for blood flow. The leaflets pivot open in response to hemodynamic pressure during the pumping stroke of the heart and then quickly pivot closed, as soon as the heart muscles relax the pumping chamber, to prevent substantial regurgitation of blood. The valve bodies, sometimes referred to as orifice rings, are principally manufactured using materials, such as pyrolytic carbon or pyrolytic carbon-coated graphite, which have sufficient resiliency to permit distension.
The valve body of a typical hi-leaflet heart valve prosthesis is generally annularly shaped, having an interior passageway with a generally circular cross-section except where a pair of diametrically opposed flat wall sections truncate the passageway at diametrically opposed secants along its periphery. The leaflets engage diametrically opposed pivot supports that are integrated within each of the flat wall sections.
The method used most commonly for installing leaflets in a valve body is to deform the valve body so that the pivot supports are spread far enough apart to permit the leaflets to be installed. For example, the valve body may be squeezed or may be distended by the application of force to interior surfaces thereof. This method is possible because pyrolytic carbon can be elastically deformed within limits, and as soon as the distending force is removed, the valve body returns to its cylindrical or annular shape, wherein the inserted leaflets are secured in the pivot supports. After the leaflets are installed, a stabilizing ring, as is well known in the art, may be shrunk-fit about the exterior surface of the valve body so as to stabilize the valve body and assure that it retains its precise generally circular cross-sectional configuration.
One specific method for installing leaflets in a valve body is to insert sets of pins within an orifice ring so that the pins engage the inside flat surfaces defining the passageway, and then spread the pins apart with sufficient forces to deform or elongate the orifice ring in order to provide clearance to install the leaflets. After the leaflets are appropriately positioned between the pivot supports, the pins are retracted so that the orifice ring returns to its original annular configuration and the pivot supports engage and secure the leaflets.
As a result of subjecting valve bodies to forces necessary to spread the pivot supports apart, the pyrolytic carbon structures or coatings of many valve bodies develop cracks. Some of these cracks are large enough to be detected with the naked eye. Valve bodies which develop such large cracks are immediately rejected; however, some of the pyrolytic carbon structures of other valve bodies have been known to develop very tiny, hairline cracks, called microcracks, which can be difficult to detect. Microcracks are undesirable because their long term effects on the structural integrity of a valve body are presently unknown. Microcracks which can be detected at the surface of a valve body generally result in rejection of the body during quality control inspection. However, some valve bodies may have microcracks beneath the exterior surface of the valve body that are not visible, and hence, are undetectable, absent destructive testing of the valve body. Consequently, there is a possibility that some heart valve prostheses having a pyrolytic carbon structure containing undetected microcracks may occasionally pass quality control inspections.
It has been felt that cracks develop in the pyrolytic carbon coatings or structures because the method by which the forces are commonly applied to the valve bodies can subject that pyrocarbon to stresses which exceed its the fracture stress limitations. Therefore, there is a need for a method for installing an occluder or a pair of leaflets into a heart valve prosthesis which does not cause cracks to develop in the pyrolytic carbon structures or coatings of such valve components.